An Israeli pilotless reconnaissance aircraft flying over Lebanon's Bekaa Valley was shot down today by Syrian surface-to-air missiles fired from within Syria near the Lebanese border, the Israeli Army command said.

Earlier, Syria claimed that its missiles shot down two Israeli aircraft but officials here insisted that only one had been lost.

It was the second Israeli-acknowledged loss drone aircraft over Lebanon since the Syrians deployed antiaircraft missiles on both sides of the border following the April 28 downing of two Syrian helicopters by Israeli warplanes. Including the two claimed today, Syria has said it downed five Israeli aircraft in all.

Today's action came 24 hours after Israeli Prime Minister Menachem Begin demanded that Syria withdraw not only its missiles in Lebanon but also those that were deployed on the Syrian side of the border after the crisis began.

Syria reacted angrily to Begin's new demand today, with the authoritative newspaper of the ruling Baath Socialist Party calling it "no less than a proclamation of war."

In Tunis, meanwhile, Arab foreign ministers opened talks on joint measures to support Syria against Israel Syrian Foreign Minister Abdul Halim Khaddam reportedly was pressing Saudi Arabia and Kuwait for full resumption of the $15 million monthly funding of Syrian truce forces in Lebanon. The funding was cut off earlier this year in a disagreement between Syria and the conservative Arab governments and it reportedly has been partially resumed.

Earlier today, special U.S. envoy Philip C. Habib, who for two weeks has been meeting with officials in the region in an attempt to avert another war, left Israel for Beirut. He was expected to go to Damascus for another meeting with Syrian President Hafe Asaad.

The last radio-controlled Israeli drone hit by the Syrians was downed on May 14, according to Israelis. The Syrians claim they shot down a drone Tuesday near the Syrian port city of Latakia.

News agencies in Beirut reported that a total of eight Soviet-built surface-to-air missiles have been fired since April 28. Israel has declared that some Syrian missiles have been fired at high-flying piloted Israeli aircraft without hitting their targets.

Today's downing underscored the growing hazards Israel faces if the diplomatic effort to resolve the confrontation fails and Begin orders an attack on the missiles in Lebanon.

Israeli military leaders, including the Army chief of staff, Gen. Rafael Eitan, have said the Syrians are steadily strengthening the defenses of the missile batteries, moving an infantry, tanks and conventional antiaircraft weapons. Concern has been expressed in Israel's parliament that a combined air strike and ground attack -- the most like assault -- would result in heavy Israeli losses.

Begin has been faulted by some critics in parliament for not attacking the missiles immediately after they were deployed, thereby lessening the risk of casualties. Begin has said he was prepared to bomb the missiles but that bad weather prevented the missions.

Meanwhile, Egypt's ambassador to Israel, Saad Mortada, met with Begin today and gave him a message from Egyptian President Anwar Sadat on the Lebanon situation. Mortada reiterated Egypt's position that all sides should show restraint and not make hasty decisions.

Mortada told Israeli radio that Egypt is optimistic that Habib's effort will succeed. He said the only "solid fact" in today's Middle East is the Egyptian-Israeli peace treaty.

Washington Post correspondent Stuart Auerbach reported from Damascus:

Western diplomats here, noting that all of Syria's confirmed hits and apparently all of their missile attacks have been on unmanned drones, suggested the Syrians are not firing on the manned warplanes Israel is also flying regularly over eastern Lebanon.

This was seen to some as an attempt to show Israel the missiles can be fired, but to avoid pushing Israel to the point where it will feel forced to attack Syrian positions to defend its pilots.

Syria attacked Begin's demand that it remove its antiaircraft missiles just inside its border as well as those in Lebanon's Bekaa Valley and that it stop firing anything at Israeli planes over Lebanon. A government spokesman said Begin's demand was new and not have been conveyed to them before by Habib.

Western diplomats here expressed dismay both over the new Israeli demand and over the way Begin presented them, while talking to television reporters, in the midst of the extremely delicate negotiations beging carried on by Habib. The U.S. envoy already is laboring under the burden of being seen as the messenger of Israeli positions rather than the bearer of American proposals.

One European diplomat called Begin's voicing of the new demands "a very rash policy" and said he has seen no signs of the "flexibility" in Begin's position that Secretary of State Alexander M. Haig Jr. credited him with recently.